In-situ processes are governed by the subsurface structure, the process fluid flows, and their interactions. A major problem with in-situ processes has been to establish intimate contact between the process fluids and the deposits, and to establish sufficient porosity or fractures to permit process fluid circulation. It is important that the fracture of the deposit is sufficiently fine so that the process proceeds at a sufficient rate with a high enough efficiency to be economic, but not too fine because fluid flow pressure drop in a packed bed increases by an order of magnitude for a bed particle size reduction of 2, thereby increasing fluid pump or compressor and piping capital and operating costs by a similar amount. There are numerous fracture techniques which are currently in use, among which are explosions, hydrofracture, leaching a soluble phase, electrofracture, abrasive cutting, and acids, to name a few.
Where these fracture techniques were applied to oil shale deposits, explosives were the most commonly used. In this potentially dangerous approach, underground tunnels were carved into the oil shale deposits in a predetermined pattern for the purpose of blasting and rubblizing the deposit. In performing the blasting process, care was required to leave sufficient support so that the entire overburden of the deposit was not collapsed into the tunnel voids. Considerable difficulty was experienced in rubblizing the oil shale deposit to produce rubble of the appropriate size which would support a reasonably uniform flame front for the retorting of the hydrocarbon values in the shale. If the rubble was not reasonably uniform and of proper size, a substantially uniform flame front was not maintained, and process and product gases mixed and reacted which contributed to the quenching of the desired retort flame front and reduced product recovery. Thus, much time and consideration was given to the blasting patterns which were used to rubblize the oil shale deposits, and even then the fracturing patterns produced were frequently by chance.